1. Field of the Disclosure
This disclosure relates to a flow discharge device and a noise attenuation device for a flow discharge device. The disclosure may be concerned with discharging bleed air in gas turbine engine, for example for bleeding air from a compressor to a bypass duct.
2. Description of the Related Art
Referring to FIG. 1, a ducted fan (or turbofan) gas turbine engine (e.g. a jet engine) generally indicated at 10 has a principal and rotational axis 11. The engine 10 comprises, in axial flow series, an air intake 12, a propulsive fan 13, an intermediate pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, an intermediate pressure turbine 18, a low-pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 generally surrounds the engine 10 and defines the intake 12, a bypass duct 22 defined by an inner wall 27 and an outer wall 26, and an exhaust nozzle 23.
The gas turbine engine 10 works in the conventional manner so that air entering the intake 12 is accelerated by the fan 13 to produce two air flows: a first airflow A into the intermediate pressure compressor 14 of the engine core and a second airflow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 14 compresses the airflow A directed into it before delivering that air to the high pressure compressor 15 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 17, 18, 19 before being exhausted through the nozzle 20 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 17, 18, 19 respectively drive the high and intermediate pressure compressors 15, 14 and the fan 13 by suitable interconnecting shafts.
During engine operation and particularly when changing rotational speed at low power it is important to ensure that the pressure ratio across each compressor 14, 15 remains below a critical working point, otherwise the engine 10 can surge and flow through the engine 10 breaks down. This can cause damage to the engine's components as well as aircraft handling problems.
To maintain a preferred pressure difference across a compressor 14, 15, or even just one stage of a compressor 14, 15, bleed assemblies 30 are provided to release pressure from an upstream part of a compressor 14,15. For example, when a gas turbine engine is operating under transient conditions, e.g. when decelerating, it may be necessary to bleed air at high pressure from the core gas flow A through the engine. Operation of a bleed assembly 30 and engine operability are described in “The Jet Engine” 6th Edition, 2005, Rolls-Royce plc, pages 79-80, and details of such operation will therefore only be briefly mentioned herein.
The flow of bleed air from the core gas flow into the bypass flow takes place over a substantial pressure drop, and can generate significant noise. It is therefore usual to provide a noise reduction device in, or at the exit of, the bleed assembly 30 between the core gas flow and the bypass duct, for example in a flow duct of the bleed assembly 30. A typical measure is to pass the bleed flow through a perforated plate, which is commonly referred to as a pepper pot. The pepper pot serves to break the single body of air flowing towards the bypass duct into a large number of smaller jets which promote small-scale turbulence and hence quicker mixing with the main flow through the bypass duct. The turbulent mixing means that there is no full recovery of dynamic head, resulting in a reduction in total pressure of the flow. In turn, this allows the velocity of the bleed flow exiting into the bypass duct to be reduced, thereby reducing noise.
In order to avoid a single large pressure drop and sudden expansion from the high pressure core flow to the bypass flow, two or more pepper pots can be used in series, in order to break the single large pressure drop into a series of smaller pressure drops.
Pepper pots are typically made from thin metallic sheets in which holes are formed, for example by laser cutting, and tend to be expensive. If a series of pepper pots are used downstream of a single bleed valve, the cost is multiplied.
Also, pepper pots are subjected to high transient pressure drops, and the shock loadings can cause them to deform or disintegrate.